Abstract

Knudsen effects, the seemingly anomalous behaviour seen in a rarified gas when the molecular mean free path becomes comparable with the size of the container, were first investigated systematically just after the turn of the century. Subsequently, closely analogous phenomena were discovered in other systems, including some which, at first sight, appear to be anything but rarified. Recent work at millikelvin temperatures in a number of laboratories has now added another two interesting examples to the list. Parpia and Rhodes of Texas A & M University have presented the clearest evidence yet for Knudsen effects in pure liquid 3He, including the observation of a so-called Knudsen minimum in the reciprocal viscosity (Physical Review Letters 51,805; 1983). Their work complements very nicely an investigation of mean free path effects in liquid lHe-4He isotopic mixtures, reported just a few months earlier by Guenault, Keith, Kennedy and Pickett (Physical Review Letters 50,522; 1983) of the University of Lancaster.